First and foremost you must install and configure the FCKEditor text editor per the instructions found in the download. 

The easiest way I have found to determine how to use some of the DNN controls is to look at the code for one of the core modules that uses the control you want to implement.  For example, if you look at the file EditFAQ.ascx which is part of the FAQ module and is located in the DesktopModules\FAQs directory of your DNN installation you will see references to "dnn:texteditor".  Below is a sample of the entries in the ASCX file.

<%@ Register TagPrefix="dnn" TagName="TextEditor" Src="~/controls/TextEditor.ascx"%>

The above line goes at the top of your ASCX file and registeres the control so it will be recognized.  The next lines of code show how you use it in the main body HTML of your module ASCX

<td>
    <dnn:texteditor ControlID="teMyDataField" id="teMyDataField" 
    runat="server" height="200" width="500" />
</td>

I hope this helps.

Chuck